The statements in this section merely provide background information related to the present disclosure and do not necessarily constitute prior art.
Quantum key distribution (QKD) systems have design considerations, the most notable of which is the maximum single travel distance of a single photon because of its attenuation while passing through an optical fiber. To increase the maximum single travel distance, signals are amplified using a quantum repeater. Ion trap is a method for implementing a quantum memory for the quantum repeater.
FIGS. 1A to 1C are diagrams illustrating the principle of a three-dimensional trap.
Ion traps are available in a number of shapes depending on the arrangement of the electrodes, including a basic form that can be implemented with a shape of field generated by four electrodes e1, e2, e3 and e4, as shown in FIGS. 1A-1C. When the electrodes e1 and e4 are grounded and a high voltage RF signal is applied to the electrodes e2 and e3 as shown in FIG. 1A, an electric field (E) is formed as shown in FIG. 1B, and the direction of the electric field (E) is continuously changed in response to the radio frequency (RF) of the applied signal. In this case, electrically charged particles are forced, on average, towards the center of the quadrangle (e.g., a square) defined by the electrodes e1, e2, e3 and e4 in FIG. 1B, when the charge amount of the electrically charged particles, the mass of the electrically charged particles, the strength of the electric field and the radio frequency satisfy certain mathematical conditions. The potential generated by such average force is referred to as a ponderomotive potential.
FIG. 1C is a diagram showing the shape of a ponderomotive potential Opp formed by the electrodes e1, e2, e3 and e4. Here, the ponderomotive potential is irrelevant to the sign of a charged particle trapped by the electrodes e1, e2, e3 and e4. The potential continues to centrally attract the charged particle despite its tendency to depart from the z-axis (FIG. 1A), but the potential does not contribute to determining the location where the charged particle may be captured along the z-axis. Therefore, in order to trap the electrically charged particle at the location as in FIG. 1A, voltage is applied to satisfy the condition of V1>V2 for a positive charge and the condition of V1<V2 for a negative charge, instead of grounding the electrodes e1 and e4.
FIG. 2A is a diagram illustrating the principle of a two-dimensional trap, and FIG. 2B is a diagram illustrating the direction of a generated electric field and the ponderomotive potential caused by the generated electric field.
High-precision fabrication of an ion trap device having a three-dimensional structure as shown in FIGS. 1A-1C is difficult to achieve and integration of multiple traps is also difficult to achieve in some situations. Therefore, for the sake of application to quantum information, the design of the ion trap is modified through a micro electro mechanical system (MEMS) process so as to allow fabrication of the ion trap device on a two-dimensional wafer surface. FIG. 2A illustrates a method of performing a conformal mapping of two-dimensional electrodes to a one-dimensional domain. By applying an RF voltage to dark sections of the circumference of a conductive circle and grounding the remainder of the conductive circle as in FIG. 2A, an electric field similar to the one illustrated in FIG. 1B is formed within the circle. As illustrated in FIG. 2A, the tangents of RF electrodes defined above are extended to form sections intersecting with an underline. Then, the RF voltage is applied to the intersecting sections and the remainder is grounded, whereby an electric field similar to the one formed within the circle is established at the location where the center of the circle is positioned. FIG. 2B illustrates the direction of the electric field generated when the electrodes are one-dimensionally arranged and the ponderomotive potential caused by the generated electric field. This is achieved by applying the RF voltage to the two dark bar type electrodes and grounding the center section between the electrodes and opposite sections outside the RF electrodes.
With an electrode structure produced using the principle described above, electrically charged particles may be captured at the triangle mark in FIG. 2B.
MEMS-based planar ion trap chips or surface ion trap chips are manufactured through a process of patterning metal electrodes on a nonconductive substrate. However, the realization of a structurally complicated ion trap is limited because of limited types of the MEMS processes available when using a nonconductive substrate. A solution to this limitation is manufacturing an ion trap chip on a silicon substrate. An ion trap chip formed on a silicon substrate generally includes a conductive film for preventing a loss of the RF voltage, a conductive film constituting RF electrodes and DC (direct current) electrodes, and an insulating layer for preventing a breakdown between the two conductive films. In case of a typical planar ion trap chip, the RF electrodes and the DC electrodes are supported by a patterned insulating layer which is inevitably exposed to at least one ion trapping position. While an ion trapping is under progress, a charge collides with the insulating layer exposed to the ion trapping position, inducing a voltage to the insulating layer which then alters the shape of the electric field and finally causes a micromotion of an ion. The ion micromotion increases a heating rate of the ion, which in turn increases escape probability of the trapped ion. Therefore, in order to achieve a more stable progress of the ion trapping, an ion trap chip needs to be constructed with no insulating layer exposed to the ion trapping position.